This invention relates to the field of antennas and more particularly to antennas for hand-held radios.
Two-way, hand-held radios have, almost exclusively, used monopole antennas utilizing the radio housing as one element or ground plane. Such antennas have been, typically, a quarter-wave whip or a physically shorter, quarter-wave helix. Since the small hand-held case is inadequate as an antenna element, the efficiency of the whip is less than that of a half-wave dipole. Helical antennas are sometimes potted in a suitable dielectric material or covered with a flexible insulative coating and have been favored for their ruggedness combined with the short lengths. However, they are inherently less efficient and, because of their short length, are strongly effected by the user's body and head, producing undesirable directivity effects. Helical antennas are difficult to tune properly and typically, after fabrication, are measured and trimmed repeatedly for the final adjustment.
Many antenna structures have been designed utilizing both rod and helix antennas. These have varied according to the application, size limitation, tuning requirements and the like. None have, however, provided a satisfactory antenna for use on a portable radio without complex structures and/or switching. Portable radios as used in security applications are typically carried on the operator's person, clipped directly to a belt or in a carrying case for belt, shoulder strap or chest carry. A long whip antenna, while desirable in certain circumstances, could be inconvenient and possibly dangerous under some emergency circumstances. A whip antenna alone could not be retracted as necessary since the radio must be operative at all times and the performance of a whip is severely degraded when greatly shortened. The ideal antenna would have a very short, but satisfactory, antenna to be used under normal conditions, but having available a reliable half-wave element for use under difficult transmitting conditions.